The invention relates to an arrangement for checking the synchronization of a receiver for data signals, having a power-versus-frequency spectrum comprising two sidebands located one on each side of a symbol frequency, in which the received data signal is sampled in each symbol interval at two synchronous instants which are half a symbol interval apart.
Arrangements of the above type are called monitoring arrangements. Synchronizing the receiver is usually effected by a clock signal device which derives a synchronous clock signal from the data signal on the basis of clock information present in the data signal itself. By means of a non-linear processing of the received data signal a strong signal component having a frequency of twice the symbol frequency can be obtained. The clock signal, which is required for signal regeneration, must be derived from this signal component by means of frequency division, which causes an ambiguity in the phase of the clock signal. A monitoring arrangement serves to determine the correct phase of the clock signal.
In a monitoring arrangement for bi-phase modulation (manchester code) described in U.S. Pat. No. 4,317,212, the monitoring criterion is derived by integrating the samples over a number of cycles of the received signal to determine whether the average power at the first sampling instant is greater than that at the second sampling instant.
From the eye patterns shown in the aforementioned patent and in FIG. 4 of the drawings accompanying this specification, it can be seen that the true eye, i.e. the eye which should correspond to the first sampling instant, contains only full amplitude excursions while the false eye is likely to contain lines which cross the eye of approximately zero voltage. Consequently, when averaged over many bit periods, the power in the true eye is greater than that in the false eye. The monitoring arrangement disclosed in the aforementioned patent uses this principle to differentiate the two eyes and effect reframing if the clock signal is incorrectly phased.
Although this arrangement is workable, there are some limitations in practice which are not apparent from a paper study of the problem. For example, it is often found that the peak amplitude of the false eye is greater than that of the true eye which erodes the power difference between the two eyes. Further, the nominally zero voltage transitions in the false eye may in fact have considerable amplitude so further eroding the power difference. Finally, the signal can contain considerable low frequency noise components due to the operation of sample and hold circuits in earlier stages of the overall system. The outcome of these defects is that it becomes progressively more difficult to distinguish between the true and false eyes as the transmission medium's attenuation is increased without resorting to excessive time constants in the integrating circuits.